The gas constant, also known as the universal gas constant, is a physical constant that appears in an equation defining the behavior of a gas under theoretically ideal conditions.

When a mole of gas is considered it follows from equation that: PV = RT

or, R = (PV)/T …. ….. …. … (1)

R is called the universal gas constant or molar gas constant. It follows from equation (1) that for one mole of any gas if the pressure is P1, volume is V1 at temperature T1 then at temperature T2 the pressure P2 and volume V2 would be such that;

(P1V1)/T1 = (P2V2)/T2

This relation is true for known quantity of any gas.

The numerical value of R may be calculated by determining the volume occupied by 1 mole of a gas at a given pressure and a given temperature. The dimensions of R may be easily deduced from the equation (1).

Example: A 1.00 L flask was filled up with a gas at pressure of 751 mm Hg temperature 26°C. What volume would this gas occupy at STP?